1. Field of the Invention
The present invention relates to an optical deflection device which can be preferably adopted in, e.g., a barcode reader or an image printing apparatus such as a digital copying machine, a printer, a facsimile apparatus, or a multifunction apparatus having the functions of these apparatuses, an image printing apparatus having the optical deflection device, and an optical deflection device manufacturing method.
2. Description of the Prior Art
An image printing apparatus or the like emits on the basis of read information a laser beam to a polygon mirror which rotates at high speed in an optical deflection device, scans a reflected beam to project it on a photosensitive body, and prints an image. Optical deflection devices using a polygon mirror are disclosed in many Japanese patent publications. One example is an optical deflection device shown in FIG. 1 (see, e.g., Japanese Unexamined Patent Publication No. 2002-48997).
An optical deflection device disclosed in this reference will be explained with reference to FIG. 1.
A polygon mirror 72 on which a reflecting surface 72a for reflecting and deflecting a laser beam is formed on a peripheral end face having a regular polygonal shape is fitted on a flange member 71 assembled integrally with an external cylinder bearing 73. The polygon mirror 72 is pressed and biased against the flange member 71 by a leaf spring 74 supported by a press plate 75, and is integrally held by the flange member 71. In this manner, a mirror unit 70 is formed.
An internal cylinder bearing 65 which is radially fitted in the external cylinder bearing 73, and an upper thrust bearing 66′ and lower thrust bearing 64 which abut against the external cylinder bearing 73 in the thrust direction are fitted on, a base member 60. The internal cylinder bearing 65, upper thrust bearing 66, and lower thrust bearing 64 are positioned in the thrust direction by a screw 68 and stationary plate 67.
A stationary yoke 61 is fixed to the base member 60, and a printed wiring board 63 having a magnet coil 62 is further fixed. A magnet 77 which faces the magnet coil 62 is fixed to the flange member 71. When the magnet coil 62 is energized, the mirror unit 70 is permitted to rotate at high speed above the base member 60 via the bearings by the interaction between the magnet coil 62 and the magnet 77.
The polygon mirror 72 is held by a holding surface 71c of the flange member 71 via a held surface 72b (processing reference surface).
To correct the face tangle angle of the reflecting surface 72a of the polygon mirror 72 to a desired value, the held surface 72b must be processed at high precision. The holding surface 71c and held surface 72b are conventionally so machined as to obtain mirror surfaces with a surface roughness (Ry) of 1 μm or less. When the mirror-finish holding surface 71c and held surface 72b abut against each other to fix the polygon mirror 72 and flange member 71, the polygon mirror 72 has a positional error due to the centrifugal force of high-speed rotation. As a result of rotating the mirror unit 70 in this state, the balance may be lost to generate vibrations.
To prevent this, the above-mentioned reference employs surface treatment for one or both of the holding surface 71c and held surface 72b so as to adjust the surface roughness (Ry) to 3 μm≦Ry≦20 μm. Note that Ry is the maximum height of undulations formed on the surface, and is defined by JIS B0601. With this arrangement, even if centrifugal force acts on the polygon mirror 72 which rotates at high speed, the polygon mirror 72 can hardly have a positional error and does not unnecessarily vibrate because of frictional force generated between the holding surface 71c and the held surface 72b. 
Recently, higher speed and higher precision of image printing are required more and more. For this purpose, the polygon mirror must also rotate at higher speed. Even if the rotational speed of the polygon mirror increases to 50,000 to 60,000 rpm, the polygon mirror must have satisfactory durability without any positional error.
Examinations made by the present inventor reveal that the invention described in the above reference can prevent a positional error of the polygon mirror when the rotational speed of the polygon mirror is up to about 50,000 rpm, but when the rotational speed exceeds 50,000 rpm, the polygon mirror may have a positional error.